Chemical modification of organic isocyanates with the isocyanate group left intact is often complicated by the high reactivity and lability of the isocyanate group. For example, it is not possible to produce isocyanatoaryl sulphonic acid esters in the same way as sulphonic acid esters free from isocyanate groups, i.e. by reacting the corresponding chlorosulphonyl aryl isocyanates with alcoholates or phenolates. If the reaction is carried out in alcoholic medium, for example, the isocyanate groups would react to form urethanes while in the heterogeneous phase, the alcoholates or phenolates, which are well known trimerization catalysts, would promote trimerization of the isocyanate groups to form the corresponding isocyanurate derivative. One way of avoiding these reactions would be by blocking the isocyanate groups with known blocking agents for isocyanate groups before such a reaction. The isocyanate groups would be reformed by thermally splitting off the blocking agent after completion of the reaction leading to the sulphonic acid ester formation. However, such a process would probably result in considerable losses of yield on account of the considerable thermal stressing involved.